Alkaline soil, sometimes referred to as “sweet” soil, has a higher concentration of base compounds measured on the pH scale. Understanding your growing environment is the first step toward successful planting. By choosing plants that naturally thrive in these conditions, you can create a vibrant landscape without constantly battling the underlying soil chemistry.
Understanding Soil Alkalinity
Soil alkalinity is defined by a pH level above 7.0. A pH of 7.5 or higher is considered alkaline, and soils above 8.5 are sometimes termed sodic, indicating a higher presence of sodium salts. This high pH often results from the geological parent material, such as significant amounts of limestone or chalk.
Alkaline conditions are also common in arid or semi-arid regions where low rainfall prevents the leaching of alkaline salts and minerals. Gardeners can identify their soil’s pH using simple at-home testing kits, or obtain more detailed results through laboratory analysis provided by local agricultural extension offices. Knowing the precise pH level is important because the scale is logarithmic, meaning a single-point difference represents a tenfold change in alkalinity.
Why Some Plants Struggle in High pH
The primary issue for many plants in alkaline soil is the unavailability of existing nutrients, a phenomenon known as nutrient lock-out. The high pH level chemically alters certain micronutrients, making them insoluble in the soil water. Since plant roots can only absorb dissolved nutrients, these elements become inaccessible.
Micronutrients like iron (Fe) and manganese (Mn) are especially prone to chemical precipitation in high pH environments. When plants cannot absorb sufficient iron, they develop chlorosis, a condition characterized by yellowing leaves while the veins remain distinctly green.
Structuring the Landscape with Alkaline-Tolerant Trees and Shrubs
Selecting species proven to handle high pH conditions is necessary when choosing large, permanent plants. Fragrant shrubs such as Lilacs (Syringa) are tolerant of alkaline soil and often perform well in slightly “sweet” conditions. Other reliable flowering shrubs include Mock Orange (Philadelphus), Deutzia, and Forsythia, which provide dense foliage and seasonal blooms.
For structural trees, the Eastern Redbud (Cercis canadensis) is an excellent choice, providing a spring display of pinkish-purple flowers. Deciduous shade trees recognized for their durability and suitability for alkaline sites include:
- Bur Oak (Quercus macrocarpa)
- Littleleaf Linden (Tilia cordata)
For year-round color, evergreens like the Colorado Blue Spruce (Picea pungens) and Junipers (Juniperus) offer striking foliage and structural mass that thrive in these conditions.
Alkaline-Loving Perennials, Annuals, and Edibles
You can add color and seasonal interest with smaller plants that prefer or tolerate alkaline soil. Excellent perennial choices known to perform well in high-pH environments include:
- Coneflower (Echinacea)
- Catmint (Nepeta)
- Dianthus (Pinks)
The tall spikes of Mullein (Verbascum) are also very adaptable, handling soil pH up to 9.0.
For annual color, robust growers that tolerate mildly alkaline conditions are Zinnias (Zinnia spp.), Cosmos, and California Poppy (Eschscholzia californica). Many common edible crops tolerate slightly alkaline soil. The cabbage family, including broccoli, Brussels sprouts, and kale, benefits from these conditions, as it helps suppress diseases like club root. Asparagus, beets, and herbs like Rosemary, Thyme, and Marjoram are also strong performers.